1. Field of the Invention
The present invention relates to a combined semiconductor apparatus useful in, for example, a light-emitting diode (LED) print head in an electrophotographic printer.
2. Description of the Related Art
FIG. 26 is a perspective view schematically showing a part of a conventional LED print unit, and FIG. 27 is a plan view showing a part of an LED array chip provided to the LED print unit of FIG. 26. Referring to FIG. 26, a conventional LED print unit 900 includes a circuit board 901 on which are mounted a plurality of LED array chips 902 having electrode pads 903, and a plurality of driving integrated circuit (IC) chips 904 having electrode pads 905. The electrode pads 903 and 905 are interconnected by bonding wires 906 through which current is supplied from the driving-IC chips 904 to LEDs 907 formed in the LED array chips 902. Further electrode pads 909 on the driving-IC chips 904 are connected to bonding pads 910 on the circuit board 901 by further bonding wires 911.
For reliable wire bonding, the electrode pads 903, 905, and 909 must be comparatively large, e.g., one hundred micrometers square (100 μm×100 μm), and the LED array chips 902 must have approximately the same thickness as the driving-IC chips 904 (typically 250-300 μm), even though the functional parts of the LED array chips 902 (the LEDs 907) have a depth of only about 5 μm from the surface. To accommodate the needs of wire bonding, an LED array chip 902 must therefore be much larger and thicker than necessary simply to accommodate the LEDs 907. These requirements drive up the size and material cost of the LED array chips 902.
As shown in plan view in FIG. 27, the electrode pads 903 may need to be arranged in a staggered formation on each LED array chip 902. This arrangement further increases the chip area and, by increasing the length of the path from some of the LEDs 907 to their electrode pads 903, increases the associated voltage drop.
The size of the driving-IC chips 904 also has to be increased to accommodate the large number of bonding pads 905 by which they are interconnected to the LED array chips 902.
Light-emitting elements having a thin-film structure are disclosed in Japanese Patent Laid-Open Publication No. 10-063807 (FIGS. 3-6, FIG. 8, and paragraph 0021), but these light-emitting elements have electrode pads for solder bumps through which current is supplied. An array of such light-emitting elements would occupy substantially the same area as a conventional LED array chip 902.